1. Field of the Invention
The present invention relates to an optical modulator.
2. Related Art Statement
It has been noted a travelling-wave type high speed optical modulator using so-called Mach-Zehnder type optical waveguide. According to this type of optical modulator, light is propagated in a pair of branched waveguide portions, respectively, so that the outputs from the branched waveguides are recombined at a recombining portion. The shift of the operational point or DC drift may occur due to a cause such as a temperature difference between the branched waveguide portions
Several methods have been proposed to control the shift of the operational point or DC drift. Representatively, Japanese patent publication P3-145, 623A discloses a method for utilizing light of substrate radiation-mode radiated from the recombining portion of the branched waveguide portions into a substrate. That is, light of on-mode is propagated in an optical waveguide and then radiated from the end face of the waveguide. Light of off-mode is radiated from the optical waveguide into a substrate as light of so-called substrate radiation-mode. An optical fiber is fitted onto the end face of the substrate to receive the light of substrate radiation-mode. The optical fiber then emits light to be detected by a photo detector. The photo detector then outputs an output signal, which is utilized for changing a magnitude of a direct current bias applied by a modulating electrode on the optical waveguide and thus controlling the operational point of the optical modulator.